1. Field of the Invention
The present invention relates to an optical scanner for forming an electrostatic latent image on a photoreceptor by scanning a laser beam, that is emitted on the basis of image data, onto the photoreceptor, and an image forming apparatus comprising thereof, and more particularly, to a technology for holding a position for image formation of the electrostatic latent image constant in the main scanning direction on the photoreceptor.
2. Description of the Related Art
Conventionally, there have been image forming apparatuses, such as copiers, printers, facsimiles, and MFPs comprising thereof, which form a color image by overlapping toner images of four colors: magenta (M), cyan (C), yellow (Y), and black (K), and transferring the color image onto a recording paper. An image forming unit in the above color image forming apparatuses is provided with a plurality of optical scanners for scanning a laser beam emitted on the basis of image data of each color onto a plurality of photoreceptor drums that are arranged so as to correspond to each color, so that an electrostatic latent image is formed on the photoreceptor drums.
Here, FIG. 3 shows a schematic configuration diagram of an optical scanner 33 according to the later-described embodiment of the present invention.
In the optical scanner 33 shown in FIG. 3, a laser beam emitted from a laser beam source 11 and scanned by a polygon mirror 12 is detected by a laser sensor 14 disposed on a scanning path of the laser beam, so as to start writing an electrostatic latent image onto a photoreceptor drum 31. In short, the optical scanner 33 decides the position of the electrostatic latent image in the main scanning direction on the photoreceptor drum 31 depending on a detection timing of the laser beam by the laser sensor 14. Such a configuration has been disclosed in, for example, Japanese Unexamined Patent Publication No. 2005-319732.
Here, as referring to FIGS. 5 and 6, one example of the laser sensor 14 is explained in details.
As illustrated in FIG. 5, the laser sensor 14 has such as a photodiode 141, an internal power source 142, a resistance 143, and a comparator 144.
The internal power source 142 converts the voltage applied from a power source not shown into a reference voltage Vr to input it into the comparator 144. The reference voltage Vr is a predetermined value for detecting that a laser beam is incident to the laser sensor 14.
On the other hand, in the laser sensor 14, when a laser beam emitted from the laser beam source 11 becomes incident to the photodiode 141, an input voltage Vi produced by the resistance 143 and the photodiode 141 is inputted into the comparator 144. In this moment, the input voltage Vi changes in accordance with the emission intensity of the laser beam incident to the photodiode 141.
And then, the comparator 144 compares the input voltage Vi from the photodiode 141 with the reference voltage Vr from the internal power source 142, and when the input voltage Vi reaches the reference voltage Vr or higher, outputs a laser beam detection signal (hereinafter referred to as “BD signal”) indicating that the laser beam has been detected.
In the laser sensor 14, the point of time when the input voltage Vi reaches the reference voltage Vr or higher changes in accordance with the emission intensity of a laser beam emitted from the laser beam source 11. Here, FIGS. 6(a) to (c) indicate the BD signals output from the laser sensor 14 in the case where the emission intensities of the laser beam source 11 are P1, P2 and P3. The emission intensities P1, P2 and P3 are: the emission intensity P1<the emission intensity P2<the emission intensity P3.
As illustrated in FIG. 6, the higher the emission intensity of the laser beam source 11 is, the higher the peak value of the input voltage Vi of the laser sensor 14 reaches, and thus, the point of time when the input voltage Vi reaches the reference voltage Vr comes earlier. Therefore, the output timings (t31, t32, and t33) of the BD signal from the laser sensor 14 therefore come earlier. In addition, the higher the emission intensity of the laser beam source 11 is, the longer the time interval during which the input voltage Vi is reaching the reference voltage Vr extends. The stop timings (t41, t42, and t43) of the BD signal output from the laser sensor 14 therefore delay.
As mentioned, in the optical scanner 33, when the emission intensity of the laser beam emitted from the laser beam source 11 changes, the output timing or the stop timing of the BD signal from the laser sensor 14 also changes. This means, the writing timing of an electrostatic latent image controlled on the basis of the BD signal also changes.
On the other hand, in the image forming apparatus, when, for example, printing conditions such as resolution of image formation and printing speed are changed, the emission intensity of the laser beam source 11 in the optical scanner 33 is sometimes changed in accordance with the changed printing conditions. In particular, when the printing speed becomes slower, the emission intensity of the laser beam source 11 is changed to be lower.
However, as mentioned above, the change in the emission intensity of the laser beam source 11 changes the output timing and the stop timing of the BD signal from the laser sensor 14. This means, the writing timing of en electrostatic latent image of the optical scanner 33 also changes. This causes a problem that the position in the main scanning direction of a electrostatic latent image to be formed on the photoreceptor drum 31 cannot be determined.
Particularly, in an image forming apparatus dealing with colors and comprising a plurality of the optical scanners 33, when the emission intensity of the laser beam source 11 is changed for each optical scanner 33 dealing with each color, a position of image formation of an electrostatic latent image by each the optical scanner 33 is misaligned in the main scanning direction on the photoreceptor drum 31. Therefore, a color deviation may occur in a color image formed when images developed on each photoreceptor drum 31 are overlapped.
The present invention has been developed on the basis of the above circumstances, with an object of providing an optical scanner, an image forming apparatus, and image forming method that allow the emission intensity of a laser beam to be changed, while at the same time, holding a position of an electrostatic latent image formed on a photoreceptor constant in the main scanning direction.